In the bulking of textile yarns having fibers of different shrinkage characteristics, such as a bicomponent yarn or yarns with blends or fibers of different shrinkage characteristics, use is made of such shrinking characteristics of the fibers under the influence of heat. On shrinkage of yarn having fibers of different shrinkage characteristics, a bulking of the yarn develops as a result of the different shrinkage behavoir, thereby yeilding so-called high-bulk yarn.
In order to produce shrinkage in the pre-drafted yarn, it is necessary to supply to each yarn a given amount of heat per unit mass. In this context, the yarn has to be maintained at the shrinking temperature for a given length of time which is a function of the material and the degree of the temperature.
In accordance with currently known yarn bulking processes (see for example German Offenlegungsschrift No. 20 39 273) the thermal energy required is supplied in wet or dry form to the yarns required to be bulked. Two basically different process variants are used for this. In one, a large volume of yarn is subjected in batches to the shrinking process (commonly known as the hank method), while the other comprises application of a continuous shrinking heat treatment to moving individual yarns, for example during a spooling and/or twisting process. A distinction is generally made between the following two variants in respect of process technology. One comprises a short period high temperature treatment of about 130.degree. C. with hot air or super-heated steam. A frequently used process for this comprises the use of radiant heaters with wall temperatures of up to 300.degree. C. The other comprises a low temperature treatment at about 100.degree. C. applied over extended periods and predominantly carried out with hot water.
These two methods have in common the circumstance that the heat is supplied to the yarn from the surface thereby rendering the poor thermal conductivity of the textile yarn the governing parameter. The efficiency of energy transfer through heating from the surroundings is hence very low. This is shown up by the feature that the wall termperature of the machine components through which the heat is transmitted is frequently very much higher than that of the yarn which, even during the high temperature treatment, is thought to barely reach the above-mentioned 130.degree. C.
Causing of this shrinkage by the use of hot water has been considered additionally beneficial when the fibers of the yarn have high-polymer molecules which have a specific affinity to water so that the water may act as an inter-molecular catalyst as a function of the existing temperature and diffusion conditions.
Accordingly, problems have heretofore existed with respect to efficient heating of yarn having fibers of different shrinkage characteristics for obtaining differential shrinkage and thus bulking of the yarns.